Displacement measurements are important in many manufacturing operations, especially those relying on feedback control. Optical sensors are particularly useful in industrial settings because they can make non-contact measurements and are relatively immune to electrical and magnetic interference.
The sensing methods considered here are limited to non-contact approaches. This excludes approaches which require mounting or printing anything on the surface to be measured, even if there is no physical contact between the mounted object or pattern and the sensor--in particular, most interferometric techniques. Some techniques most comparable to the inverse-square sensor, with a range of a few millimeters to a few meters, 1 percent or better resolution, and measurement rates of at least a few hundred Hertz, are discussed below. Sensing schemes may be divided into non-optical and optical techniques. Non-optical techniques include ultrasound, capacitive, magnetic and eddy current. Optical techniques include sensitive volume, fiber surface reflection, triangulation, speckle, focus-contrast detection and echo ranging. A comparison of the important characteristics of each of these methods follows.
Ultrasonic range sensors (Dean Campbell, "Ultrasonic Noncontact Dimensional Measurement," Sensors, Vol. 3, No. 7, pp. 37-43, 1986) emit a pulse of high-frequency acoustic energy and measure the time for an echo to return from the measurement surface. Resolutions of .+-.25 .mu.m out to 50 cm or .+-.2 cm up to 10 m away can be achieved by commercially available devices. The transducer's spot size is large (typically .gtoreq.15 mm, even at close range), restricting resolution for surfaces with texture directional, reducing the returned pulse amplitude and thus sensitivity for inclined surfaces. A major problem is the variation of the speed of sound with air temperature; a change of 6C near room temperature causes a 1 percent change in the sound speed, with a proportional change in the measured distance. Calibration schemes employing a fixed reference object near the sensor can help with steady temperature drift, but thermal gradients between the sensor and the surface remain a problem. The sampling speed of these sensors is limited by the speed of sound; for example, a round trip to 50 cm requires 3 ms.
Electrical sensors such as capacitive, hall effect, and eddy current sensors generally require conductive or magnetic surfaces, which greatly restricts their applicability. Many commercial models are available, with a wide variety of ranges and resolutions.
The sensitive volume technique (Antal K. Bejczy, "Smart Sensors for Smart Hands," Paper 78-1714, AIAA/NASA Conference on "Smart" Sensors, Hampton, Va., Nov. 14-16, 1978) is similar to the inverse-square sensor only in that both are reflection-based amplitude sensing schemes. Illumination and collection optics look at intersecting volumes of space in front of the sensor. Illumination light is scattered into the collection optics when a surface is within the intersection volume. The quantity of light collected is a function of the distance to the surface and also of surface properties and illumination power. Useful range is up to about 8 cm, depending on sensor geometry. Construction of this sensor is somewhat simpler than the inverse-square sensor, but the results are surface dependent.
The fiber surface reflection sensor (D. E. N. Davies, J. Chaimowicz, G. Economou, and J. Foley, "Displacement Sensor Using a Compensated Fiber Link," in R. Kersten and R. Kist, eds., Second Intl. Conf. on Optical Fiber Sensors. Sept. 5-7, 1984, pp. 387-390; Charles M. Davis, "Fiber Optic Sensors: an Overview," in Fiber Optic and Laser Sensors II, Proc. of SPIE Vol. 478, pp. 12-18; Gregory Hull-Allen, "Reflectivity Compensation and Linearization of Fiber Optic Probe Response" in Optical Systems Engineering IV, Proc. of SPIE Vol. 518, pp. 81-85; Leo Hoogenboom, G. Hull-Allen and Steven Wang, "Theoretical and Experimental Analysis of a Fiber Optic Proximity Probe," in Fiber Optic and Laser Sensors II, Proc. of SPIE Vol. 478, pp. 46-57; N. E. Lewis, M. B. Miller, W. H. Lewis, "Fiber Optic Sensors Utilizing Surface Reflections," in Fiber Optic and Laser Sensors II, Proc. of SPIE Vol. 478, pp. 39-45) is also reflection based. One or more pairs of optical fibers are mounted next to each other, with light emerging from one of the pair. In the most common arrangement, the fibers are within about a fiber diameter of a surface, and the amount of light gathered by the collection fiber is a function of the distance to the surface. Several compensation schemes have been proposed to eliminate dependence on surface properties. The useful range for this sensor is only up to about 2 mm, with less than 0.1% error.
Laser triangulation determines distance by measuring the apparent lateral position of a spot of light projected onto a surface at an oblique angle (James T. Luxon and D. E. Parker, Industrial Lasers and Their Applications, Prentice-Hall, pp. 154-157). Many configurations of the light source and detector are used; the most accurate approaches use several sources and detectors, which can also provide surface orientation information (M. Fuhrman and T. Kanade, "Design of an Optical Proximity Sensor
Using Multiple Cones of Light for Measuring Surface Shape," Opt. Eng. 23 (5) 546-553, 1984). Accuracies of .+-.50 .mu.m over 5 cm or .+-.1 .mu.m over 2 mm at rates of several kHz are quoted in product literature ("Optical Systems Featured," Sensor Review Vol. 5, No. 3, pp. 172-174, 1985).
Several schemes for sensing distance with speckle have been used (Atul Jain, "System for relative motion detection between wave transmitter-receiver and irregular reflecting surface," U.S. Pat. No. 4,210,399, 1/10/77; Akihiro Hayashi and Yoichi Kitagawa, "High-resolution rotation-angle measurement of a cylinder using speckle displacement detection," Appl. Opt Vol. 22, No. 22, pp. 3520-3525, 1983; Nobukatsu Takai and Toshimitsu Asakura, "Displacement measurement of speckles using a 2-D level crossing technique," Appl. Opt Vol. 22, No. 22, pp. 3514-3519, 1983). Most measure relative displacement, and require averaging many speckle cells in order to achieve reasonable accuracy. These techniques have not been proven in industrial settings.
The focus contrast sensor forms the basis of the autofocus feature found on some cameras. In one approach suited to industrial displacement measurement (Distance "Sensing Uses Automatic Focusing Technology," Sensor Review Vol. 4, No. 4, pp. 172-173, 1984), a row of light detector pairs looks out through a lens at the subject to be ranged. The detectors measure the local contrast in the scene at the focal plane and from this a microprocessor determines the distance between the subject and the front focal plane. An accuracy of about 5 percent is typical for a fixed-focus lens, but this can be substantially increased with a variable focus lens. The main drawback of this approach is the need for a subject with substantial optical contrast.
Echo ranging with light is similar to ultrasound: a short pulse of light is directed at the surface to be ranged, and the travel time for the reflected pulse to return gives the distance. Because light travels at about 3 ns per meter, very fast and expensive electronics and light sources are required to provide sub-meter resolution. This technique is best suited for the 10 m to 1 km range, with .gtoreq.0.5 m resolution.
The two techniques with range and resolutions most comparable to the inverse-square approach of this invention are ultrasonic ranging and laser triangulation. Ultrasound has similar range and resolution, but a larger beam diameter and poor performance on tilted surfaces. It is also sensitive to environmental effects. Laser triangulation can produce better resolution over a given range, but the sensor is more expensive and complicated. The inverse square scheme of this invention is much less expensive in systems where multiple range measurements must be made simultaneously, since one light source and detector can service a large number of sensors, and the incremental cost of adding each sensor is small.
An integrated optical sensing system architecture can meet many industrial sensing needs (G. Kychakoff, P. H. Paul and R. K. Hanson, "Fiber Optic Sensor System for Industrial Monitoring Applications," in Technical Digest of the Conference on Lasers and Electrooptics, 1984, Optical Society of America, Wash. D.C., pp. 132-133). Light from a single light source is directed by an electrooptic multiplexer to a number of sensor locations via optical o fibers. After interaction with the industrial process, the returned light is demultiplexed and preprocessed before computer acquisition. This scheme permits a single light source and detection system to access multiple points simultaneously, and keeps delicate optical components away from the factory environment. It also provides for simple extension or modification of the sensors and sensing system.
It is a primary objective of this invention to provide an improved non-contact displacement sensor. More particularly, it is an objective herein to provide an improved non-contact sensor having a reasonably long range and high resolution.
A further objective herein is to provide a displacement sensor which is useful for studying tilted or rounded surfaces.
In summary, this invention comprises a simple, inexpensive optical displacement sensor that uses the inverse-square attenuation of light reflected from a diffused surface to calculate the distance from the sensor to the reflecting surface. More particularly, light emerging from an optical fiber or the like is directed onto the surface whose distance is to be measured. The intensity I of reflected light is angle dependent, but within a sufficiently small solid angle it falls off as the inverse square of the distance from the surface.
In a preferred embodiment, at least a pair of optical detectors are mounted to detect the reflected light within the small solid angle, their ends being at different distances R and R+.DELTA.R from the surface. The distance R can then be found in terms of the ratio of the intensity measurements and the separation length as ##EQU2## The approach disclosed herein reduces dependence on surface properties and illumination power, and hence, eliminates the need for elaborate calibration of the sensor.
The sensor may be designed to form part of a comprehensive optical sensing system. A single light source can be used to drive a large number of the sensors. Arrays of these devices to sense curvature or orientation of the surface can be built easily and inexpensively. The device is thereby particularly suited to applications which require simultaneous displacement measurements at multiple locations.
Although this invention is described with respect to the use of a laser as the light source, other sources may be used efficiently, preferably being highly collimated and nearly monochromatic. Also, the use of fiber optics is not required as other detectors may be efficiently used.
A preferred form of this invention will be described with reference to the following figures.